Lithium secondary batteries, which exhibit high energy density and operating potential, have long cycle lifespan and low self-discharge rate. Due to such characteristics, lithium secondary batteries are broadly used in mobile devices, and medium and large-scale devices such as electric vehicles (EVs), hybrid electric vehicles (HEVs), power storage devices and the like.
As cathode active materials of lithium secondary batteries, lithium-containing cobalt oxides such as LiCoMO2 are mainly used. Besides the lithium-containing cobalt oxides, use of lithium-containing nickel oxides such as LiNiMO2, lithium manganese oxides such as LiMnMO2 having layered structures and lithium manganese oxides such as LiMn2MO4 having a spinel structure is being considered. Recently, LiMO2 where M is Co, Ni and Mn is being used.
During a synthesis process, a large amount of Li by-products is generated on a surface of the LiMO2 where M is Co, Ni and Mn. Most of the Li by-products consist of Li2CO3 and LiOH, and, as such, a cathode paste may be gelated during a cathode paste manufacturing process and, after manufacturing an electrode, gas may be generated by charging and discharging.
Meanwhile, in lithium secondary batteries, an electrolyte is an essential element as an intermediate for ion delivery. The electrolyte generally consists of a solvent and a lithium salt. As such, LiBF4, 6LiPF6 and the like as lithium salts are mainly used in view of solubility, chemical stability and the like. However, an electrolyte manufactured by dissolving a lithium salt including fluorine (F) as described above reacts with a small amount of water in the electrolyte and, as such, hydrofluoric acid (HF) is generated and the generated hydrofluoric acid may decompose an electrode.
In addition, lithium carbonate (Li2CO3) is not eluted in a water-insoluble and pure solvent and exists stably. However, when lithium carbonate (Li2CO3) is reacted with hydrofluoric acid (HF), the lithium carbonate (Li2CO3) is eluted in an electrolyte and thereby carbon dioxide (CO2) is generated. Accordingly, a large amount of gas is generated during storage and cycling. As a result, swelling of batteries may be induced and high temperature stability may be deteriorated.